Within the power industry, valves are operated remotely from open, closed and intermediate positions to improve or maintain utility power plant output, or in many cases to provide for the protection of plant equipment, as well as protection of the general public from release of radioactive materials either directly or indirectly. Continual, proper operation of the valves is essential to the well-being of the industry and the general public. The extreme emphasis on safety in Nuclear power plants (and the presently bad reputation of the nuclear industry) has put a premium on the importance of maintaining proper operation of valves, of which there may be hundreds within a single plant.
At the forefront of industry attempts to monitor and maintain proper operation of these critical valves is the recent invention of Arthur G. Charbonneau, et al described in U.S. Pat. No. 4,542,649. The Charbonneau, et al invention disclosed a new and important valve operator and monitoring system to measure, record and correlate valve stem load, limit and torque switch positions, spring pack movement and motor current providing time related information on valve performance. The information made available by Charbonneau, et al provides a direct indication of developing valve and operator problems such as excessive or inadequate packing load, excessive inertia, proximity to premature tripping, incorrectly set operating limit and torque switches, improperly functioning thermal overload devices, inadequate or excessive stem thrust loads, gear train wear, stem damage, and load relaxation.
The prior, Charbonneau et al invention teaches the use of a limit/torque switch position indicating device. This device provides data, for recording and correlation with the other collected data, which indicates when, in relative time, a particular limit switch or torque switch is open or closed. The particular sensing circuit of Charbonneau et al has been found to be impractical and unusable in certain industry applications. For example, the Charbonneau et al circuit requires access to a negative terminal for a ground. In most all nuclear power plants known to the industry, regulations prevent lifting wire leads, or cutting into or splicing into wiring systems of the valve operator to gain access to a negative terminal; and in many nuclear power plants there are no free negative terminals available in the operator's control circuitry. Furthermore, Charbonneau et al makes use of the current flowing through the operator limit and torque switches to induce a field in a coil. Such a method can not be employed in power plants where the operator switches are found in DC circuits, since DC will not induce a field in a coil.